1. Field of the Invention
The present invention relates generally to a communication system, and more particularly, to an intra-refreshing control method and apparatus for efficiently processing image quality of video data in a video telephone communication system.
2. Description of the Related Art
The Wideband Code Division Multiple Access (WCDMA) scheme, a wireless access scheme used for the 3rd generation mobile communication system, aims at supporting broadband services while providing high Quality of Service (QoS). The WCDMA scheme is being developed with the application of the communication schemes for supporting high-speed Internet access, high-speed image transmission, and high-quality packet data transmission. There are several possible schemes proposed for performing video telephony over the WCDMA communication system.
In the video telephony system, a video encoder compresses an input image in two different frame types. A frame obtained by compressing a one-sheet image independently is called an ‘I-frame’, while a frame obtained by compressing only the difference between the current image and the previously encoded and decoded frame is called a ‘P-frame’. Generally, the I-frame is 5 to 10 times greater than the P-frame in size. The video telephony system repeatedly performs a communication process of transmitting several tens or hundreds of P-frames after transmitting one I-frame.
In the video telephony system, new image quality recovery schemes are proposed to recover the image quality of video data when the received video data suffers a loss or an error, i.e., when it suffers from a transmission error. One of the recovery schemes includes an intra-refreshing scheme in which a receiver requests a transmitter to insert and transmit an I-frame before its original period.
FIG. 1 is a diagram illustrating basic architecture of a video telephone communication system to which the present invention is applicable. Generally, the video telephony communication system includes a transmitter (TX), a receiver (RX), and networks that exchange video data with the transmitter and the receiver.
Referring to FIG. 1, the transmitter includes a video encoder 101, a segmentation block 103, an error detection block (or CRC Attach block) 105, and a packetization block 110.
The video encoder 101 compresses the image that the transmitter intends to transmit. The segmentation block 103 segments the compressed image according to the transmission unit of the network. The error detection block 105 attaches an error detection code, or Cyclic Redundancy Check (CRC), used for detecting a transmission error, to the compressed and segmented video data. The packetization block 110 attaches a sequence number (S/N) indicating a sequence of the packet to the CRC-attached video stream to make a video packet stream suitable for transmission, and transmits it to a network 120. The video packet stream is transferred to the counterpart receiver such as the video telephone set or the communication apparatus, over the network 120 composed of various wired/wireless communication channels.
The receiver includes a de-packetization block 130, an error detection block (or S/N and CRC Check block) 145, a concatenation block 143, and a video decoder 141. The depacketization block 130 de-packetizes the video packet stream received over the network 120 into a video data part, a CRC part, and an S/N part.
The error detection block 145 generates a CRC of a video stream, and compares the generated CRC with the existing CRC. Further, the error detection block 145 checks an S/N, and determines whether the S/N is greater than the expected value. If the generated CRC is inconsistent with the CRC of the transmitted video packet, the error detection block 145 considers that the received video data includes an error. In addition, if the checked S/N is greater than the expected S/N, the error detection block 145 considers that a particular video packet has not arrived at the receiver as it is lost during its transmission.
In other words, if the generated CRC is consistent with the CRC of the transmitted video packet and the checked S/N is also equal to the expected value, it means that the received video packet has arrived as expected without error during its transmission, so the concatenation block 143 concatenates the received video data to the previously arrived video data, and transfers the resulting video data to the video decoder 141. The video decoder 141 decodes the video stream, providing the original video image. The video decoder 141 displays the decoded video image on the receiver, such as the video telephony set or communication apparatus.
However, if the two CRCs detected in the error detection block 145 are inconsistent with each other or the checked SN is greater than the expected value (S/N JUMP), the error detection block 145 immediately sends, to a controller 150, a signal indicating that an error is included in the received video data or the packet has not arrived at the receiver as it is lost in the network.
The controller 150 of the receiver transmits a signal indicating the presence of an error in the video data received at the network 160, or indicating the reception failure, to the transmitter over a wireless channel. Therefore, the network 160 transmits the error detection signal provided from the controller 150 of the receiver, to a controller 170 of the transmitter.
The controller 170 of the transmitter sends to the video encoder 101 a notification indicating the error occurred in the transmitted video data, thereby allowing the transmitter to recover the damaged image quality, i.e., to perform intra-refreshing. That is, at the request of the receiver, the transmitter immediately compresses/inserts/transmits an I-frame before its original period.
In other words, the controller 150 of the receiver sends a request for intra-refreshing to the controller 170 of the transmitter, and upon receipt of the intra-refreshing request signal from the receiver's controller 150, the controller 170 of the transmitter forwards it to the video encoder 101. Then the video encoder 101 of the transmitter immediately inserts an I-frame regardless of the state of the transmission frame, thereby allowing the receiver to stop the spread of the error and to recover the image quality.
Conventionally, as described above, the receiver sends an intra-refreshing request to the transmitter every time it detects that the received video data suffers a loss or an error. Then the transmitter compresses and transmits an I-frame every time in response to the intra-refreshing request from the receiver.
FIG. 2 is a diagram illustrating a conventional timing diagram for a description of a possible problem of intra-refreshing.
In the video telephony communication system, an arbitrary user A is assumed to be a transmitter and an arbitrary user B is assumed to be a receiver. If the receiver, or user B, checks the received video data and determines that the transmitted data has suffered a loss or an error, it sends an intra-refreshing request to the user A, or transmitter. That is, the receiver requests the transmitter, or user A, to transmit an I-frame obtained by compressing the entire image. However, if the receiver, or user B, detects occurrence of another error before the I-frame arrives from the transmitter, or user A, in response to the intra-refreshing request, the receiver sends again an intra-refreshing request.
Referring to FIG. 2, at time 210, the user B's receiver, which has detected that a received frame Pm has suffered a loss or an error, sends a request for transmission of an I-frame to the user A's transmitter to recover the image quality. In this case, the user B's receiver transmits a Video Fast Picture Update Command (VFPU) signal to the user A's transmitter.
Thereafter, before receiving an I-frame from the user A's transmitter, or a response signal to the VFPU signal transmitted at time 210, the user B's receiver detects a loss or error of another video data, and sends again an intra-refreshing request therefore at time 230. That is, the user B's receiver transmits a VFPU signal for another loss or error to the user A's transmitter.
At time 215, the user A's transmitter receives the VFPU signal transmitted at time 210 from the user B's receiver, and transmits an I-frame 220 at time 220. Thereafter, at time 235 where the user A's transmitter has transmitted several P-frames after transmitting the I-frame 220, the user A's transmitter receives again a VFPU signal 230 from the user B's receiver. Then, at time 240, the user A's transmitter transmits an I-frame 240 in response to the intra-refreshing request sent at time 230.
In this way, the user B's receiver frequently sends an intra-refreshing request according to the error detection, and the user A's transmitter frequently transmits the I-frame in response thereto.
As the I-frame 220 arrives without error in response to the first intra-refreshing request due to the packet loss or error, the packet loss or error is fully recovered, but is the second I-frame 240 does not contribute to the image quality. In this state, the transmitter frequently transmits the large-size I-frame, which causes a reduction in the total number of image frames actually received at the receiver. That decreases, from the standpoint of the receiver, the frame rate. Generally, the I-frame has a larger size, but is not higher than the P-frame in image quality.
FIG. 3 is a diagram illustrating a conventional timing diagram for a description of another possible problem of intra-refreshing.
Referring to FIG. 3, the receiver detects the occurrence of data loss or error in the previously received video data and sends an intra-refreshing request 260 to the transmitter according to the detection, and a compressed I-frame 250 periodically arrives without a loss of the video data before an I-frame 270 arrives from the transmitter, thereby fully recovering the lost image.
In this case, though the receiver has sent an intra-refreshing request 260, 265 to the transmitter, the intra-refreshing requested I-frame 270 does not always contribute to the quality improvement based on the image recovery. Unfortunately, however, the sending of the intra-refreshing request 260 causes a mere decrease in the number of image frames actually received at the receiver.
As described above, the conventional scheme sends an intra-refreshing request without comprehending the processing state of the video encoder in the counterpart for the received video data, i.e., in the counterpart transmitter or receiver, causing unnecessary transmission of the command signal. The unnecessary transmission of the I-frame due to the intra-refreshing causes a fatal problem that decreases an average transmission rate of the video frames.